I would suggest that asking a mentor to sign an NDA is inappropriate. You are already asking the mentor to do you a significant favor by providing ongoing advic

I would suggest that asking a mentor to sign an NDA is inappropriate. You are already asking the mentor to do you a significant favor by providing ongoing advice and guidance. To then ask a mentor to sign a legal document is placing an unfair burden on the mentor. The two solutions are to either (a) find a mentor you can trust with confidential information or (b) keep your "secret sauce" information private until you have created others barriers to entry.